DANVILLE, Ky., Oct 11 (Reuters) - Democrats are betting on U.S. Vice President Joe Biden to deliver a feisty performance Thursday evening during a high-stakes debate against Republican challenger Paul Ryan and seize back momentum in the race for the White House.

Republican Mitt Romney's steady climb in polls since President Barack Obama's poor showing in their first debate last week have intensified expectations for the vice presidential showdown with less than four weeks before the Nov. 6 election.

The former Massachusetts governor has taken the lead in national surveys and narrowed the gap by which he trails Obama in many of the swing states that will decide the election.

Romney led the Democratic incumbent by 47 percent to 44 percent in the Reuters/Ipsos daily tracking poll on Thursday - the online poll showed the challenger with a one percentage point advantage on Wednesday.

Several new swing-state polls released on Thursday also showed Romney edging closer to Obama among likely voters. Obama had a six-point lead in an Ohio poll and a five-point lead in a Virginia survey. Separate polls in Virginia, Colorado, Florida and Wisconsin had gaps of three points or less.

Biden, 69, a veteran politician and accomplished debater known for delivering withering attacks with a smile, will try to improve the outlook for the Democrats before Romney and Obama meet for a second televised debate next Tuesday.

The 42-year-old Ryan, who has served seven terms in the U.S. House of Representatives and is chairman of the House Budget Committee, has been criticized for being a cautious campaigner since Romney chose him as his running mate in August. The Wisconsin congressman will look to ward off Biden's attacks while avoiding his own tendency to get mired in numbers.

Foreign policy is expected to be a ripe topic. Republicans are eager to take the Obama administration to task over last month's attacks on U.S. diplomatic missions in the Middle East, and Biden will be ready to point out the lack of foreign policy experience on the Romney-Ryan ticket. ï¿½ Biden is expected to challenge Ryan on his and Romney's assertions on taxes, health care and other key issues, something that Democrats felt Obama largely let slide during the debate last week.

"This has turned into a legitimate high-stakes debate because the ground has shifted so profoundly on the Democrats," said Cal Jillson, a political scientist at Southern Methodist University in Texas.

"Biden at least has to hold his own so panic doesn't set in for Democrats," he said. "They don't want to lose two in a row."

Biden and Ryan meet at 9 p.m. EDT (0100 GMT on Friday) in the nationally televised debate from Centre College in Danville, Kentucky.

MODERATE MITT

Romney has shifted more toward the political center on abortion, taxes and other hot-button issues, starting with last week's debate, and thousands of people have turned out for him at campaign appearances since.

Democrats have accused the Republican, who called himself a "severe conservative" while vying for his party's presidential nomination, of shifting or misrepresenting his positions. Biden is expected to be more confrontational than Obama in an encounter that covers both domestic and foreign policy.

"It's a great opportunity, again, to drive the distinctions between the two candidates and talk about distinct approaches to the future," Obama campaign deputy manager Stephanie Cutter said on MSNBC's "Morning Joe" program.

"Right now the Romney campaign is running away from some of their positions like unwanted stepchildren, but we're going to hold them to them and explain to the country exactly what the differences are here because the choice is very stark," she said.

As he boarded Air Force Two on Thursday morning to fly to Kentucky for the debate, Biden told reporters he was ready for the face-off. "I'm looking forward to it," he said.

'JOE JUST NEEDS TO BE JOE'

Biden, the former chairman of the Senate Foreign Relations and Judiciary committees, has more experience on the national stage than Ryan.

He was a strong performer in the Democratic primary debates during his failed 2008 run for the White House and fared well against Republican Sarah Palin in that year's vice presidential debate.

But he also has a reputation for gaffes, including a recent remark that the middle class has been "buried for the last four years" - almost the span of Obama's presidency - by a bad economy.

Obama said he was not worried about Biden.

"I think Joe just needs to be Joe. Congressman Ryan is a smart and effective speaker. But his ideas are the wrong ones and Joe understands that," Obama said in an interview with ABC News on Wednesday.

Ryan's previous debate experience consists of congressional contests in Wisconsin. He was happy to raise expectations for Biden's performance.

"Sure it's a nervous situation. Joe Biden's one of the most experienced debaters we have in modern politics," Ryan told reporters. "But the Achilles' heel he has is President Obama's record."

Ryan's budget plan, which has made him a hit with conservatives, is likely to play a starring role in the debate. Ryan proposes slashing government spending and creating a "voucher" system for the Medicare healthcare program for seniors, which Democrats say would leave some retirees paying more of their medical costs.

"The challenge for Biden, and Obama didn't do this at all, is to put the other side on the defensive and make them explain themselves and their policies," said Steven Schier, a political scientist at Carleton College in Minnesota.

Biden said he has been studying Ryan's plan during his debate preparations. Democratic U.S. Representative Chris Van Hollen has played Ryan in mock debates, while Ryan has been prepared by former U.S. Solicitor General Ted Olson.

(Additional reporting by Susan Heavey in Washington and Jeff Mason in Delaware; Writing by John Whitesides and Patricia Zengerle; Editing by Alistair Bell, Karey Wutkowski and Paul Simao)